Derating

[A/A Fuel GTX]

If I must derate for two reasons such as temperature and number of current carrying conductors, where does the code address this issue? For example, THHN, 110 degree ambient and 4 current carrying conductors.

 

[charlie b]

Re: Derating

The impact is cumulative. First you multiply by 87% (for temperature), and then you multiply by 80% (for number of conductors). You could do it in the other order, if you prefer, but you do have to use both factors.

 

[A/A Fuel GTX]

Re: Derating

So for example if I were using a 500kcmil THHN CU conductor, I would multiply 430 X.87 which equals 374A and then multiply 374 X .80 which equals 299A. In which case I'd use a 300A OCPD.

 

[pete m.]

Re: Derating

Two code sections come to mind.
Table 310.15(b)(2)(a) and the ambient temperature adjustments found at the bottom of table 310.16 for conductors in a raceway.

Pete

 

[charlie b]

Re: Derating

Originally posted by m73214: So for example if I were using a 500kcmil THHN CU conductor, I would multiply 430 X.87 which equals 374A and then multiply 374 X .80 which equals 299A. In which case I'd use a 300A OCPD.

Exactly so.

 

[1793]

Re: Derating

So, once we get the final number, 299, do we then use 60 degree or the 75 degree for the OCPD?

 

[infinity]

Re: Derating

Originally posted by 1793:
So, once we get the final number, 299, do we then use 60 degree or the 75 degree for the OCPD?

If your equipment had terminations rated for 60 degrees you would use the 60 degree column, 75 degrees the 75 degree column. Your adjusted ampacity can't be higher than the ampacity listed in the column that applies to the rating of the lugs on your equipment.

 

[1793]

Re: Derating

Thanks. That is what I was looking for.

[ February 21, 2006, 05:35 PM: Message edited by: 1793 ]

 

[jim dungar]

Re: Derating

The temperature rating of the equipment decides which column to use. 110.14(C)(1)(b) requires conductors rated 75C on devices above 100A.

 

[RUWired]

Re: Derating

Norb, 75 degree column.(110.14(C)1(B).Devices over 100 amps ,and larger than 1/0awg use the 75* column.
Rick

 

[infinity]

Re: Derating

Originally posted by RUWired:
Norb, 75 degree column.(110.14(C)1(B).Devices over 100 amps ,and larger than 1/0awg use the 75* column.
Rick

Rick brings up a good point. Over 100 amps and larger than #1 requires the use of 75 degree terminations. I would add that most equipment today has lugs rated for 75 degrees. Even the lugs on conductors smaller than 100 amps.

[ February 21, 2006, 06:59 PM: Message edited by: infinity ]

 

[1793]

Re: Derating

I think I have found the answer to "Need a little help..." in this thread.